Nucleic acid amplification reactions are well known and are employed to increase the concentration of a target nucleic acid in a test sample. The “target nucleic acid” typically is present in a sample in low concentrations and therefore cannot easily be detected without amplifying it to increase the concentration of the target sequence in the sample. The polymerase chain reaction (PCR) is one nucleic acid amplification reaction commonly employed for purposes of amplifying a target nucleic acid sequence.
According to the principles of PCR, “primer sequences” are used to prime synthesis of copies of the target sequence. Specifically, under appropriate conditions, primer sequences hybridize to opposite strands of a double stranded nucleic acid sequence such that the primers flank the target sequence. Once hybridized, the primers are extended using enzymes such as, for example, DNA polymerase which extend the primer sequences to thereby generate copies of the target sequence. Additional copies of the target sequence are generated by cycling the above steps of (i) hybridizing and extending the primer sequences and (ii) dissociating the extended primer sequences (or copies of the target sequence) so that additional primers can hybridize to the original target, as well as copies of the target sequence. Hence, multiple copies of the target sequence are generated.
Once amplified, copies of the target sequence can be detected to determine if the target sequence originally was present in the test sample. Of course, if the target sequence was not present, amplification should not occur and the target sequence should not be detected. In any event, amplified target sequences are typically detected using labels. Labels are moieties that have a detectable property and can be incorporated into the copies of the target sequence. Labels typically are incorporated into the amplified target sequences by attaching the labels to primer sequences that are then incorporated into the amplification product as specified above. Alternatively, for example, extension products can be labeled by incorporating labeled nucleotides into such products during primer extension. The presence of the target sequence in the test sample can then be determined by detecting the labeled amplification product.
Amplified target sequences also can be detected using labeled probes that hybridize to a strand or both strands of an amplified target sequence. However, it is sometimes desirable to employ a probe that hybridizes to only one strand of a double stranded amplification product. The effect of such a detection scheme, at least as it applies to a double stranded target sequence, is that a single strand of amplified target sequence is detected to determine the presence of the target sequence in the test sample. However, detecting a single strand of an amplification product can be inefficient insofar as the signal plateaus and sometimes drops (or hooks) as the number of target sequences originally present in the test sample increases. Alleviating the “hooking” or “plateauing” phenomenon and providing a linear signal over a broader range of target sequence concentrations would be beneficial, especially for amplification based assays designed to quantify the amount of a target sequence in a test sample.
It would be expected that substantially increasing the concentration of one primer over the other would alleviate this problem by generating more of the sequence that is detected. Indeed, U.S. Pat. No. 5,066,584 describes a method for preferentially generating one strand of a double stranded target sequence by vastly increasing the concentration of one primer. However, this requires excess reagents and therefore excess costs associated with preferentially producing one of two single strands. Additionally, substantially increasing primer concentrations may increase the chances of non-specific priming and therefore amplification of non-target sequences. Moreover, many times, competing non-specific reactions will interfere with the efficient amplification of the sequence of interest. Therefore, it may be expected that substantially increasing the concentration of one primer over the other primer may present problems in amplification assays designed to be of high sensitivity (i.e. designed to detect low numbers of a sequence of interest).